Method for providing information of access point selection

ABSTRACT

A method for providing information of access point (AP) selection in a wireless local area network (WLAN) system, the method including receiving from candidate APs the AP selection information including channel correlation information for other stations associated with the candidate APs; and selecting one AP from candidate APs according to the channel correlation information corresponding to the candidate APs. Accordingly, a gain of multi-user multiple input multiple output (MIMO) can be increased, and an AP suitable for a terminal can be selected.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/376,109 filed on Jan. 5, 2012, which is a National Phase of PCTApplication No. PCT/KR2009/005424 filed on Sep. 23, 2009, which claimspriority under 35 U.S.C. §119(e) to U.S. Provisional Application Nos.61/183,557 filed on Jun. 3, 2009, and 61/218,982 filed on Jun. 21, 2009,and under 35 U.S.C. §119(a) to Patent Application No. 10-2009-0064931filed in the Republic of Korea on Jul. 16, 2009, all of which are herebyexpressly incorporated by reference into the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless local area network (WLAN),and more particularly, to a method for providing a terminal withinformation of access point (AP) selection in a WLAN system, a methodfor selecting an AP, and a station for supporting the methods.

2. Discussion of the Related Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted.

In the initial WLAN technology, a frequency of 2.4 GHz was usedaccording to the IEEE 802.11 to support a data rate of 1 to 2 Mbps byusing frequency hopping, spread spectrum, infrared communication, etc.Recently, the development of a wireless communication technique appliesorthogonal frequency division multiplex (OFDM) or the like to the WLANto support a data rate of up to 54 Mbps.

In addition, the IEEE 802.11 is developing or commercializing standardsof various technologies such as quality of service (QoS) improvement,access point (AP) protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc.

In the WLAN system, a terminal used as a non-AP station performs ascanning process to find an accessible AP. The scanning process is aprocess in which the terminal obtains information on each AP and a listof candidate APs to which the terminal is associated in a subsequentprocess (i.e., an association process) so that the terminal becomes amember of a specific extended service set (ESS).

If terminals having a high channel correlation are associated with thesame AP, it is difficult for the AP to provide a service simultaneouslyto the plurality of terminals having a high channel correlation.Therefore, when a terminal selects an AP, it is advantageous to selectan AP not associated with a terminal having a high channel correlation.The scanning process may be used in order for the terminal to obtaininformation indicating whether other terminals having a high channelcorrelation are associated with a certain AP. By performing such ascanning process, the terminal can further effectively select the AP.

SUMMARY OF THE INVENTION

The present invention provides a method of selecting an access point(AP) in a wireless local area network (WLAN) system capable of usinginformation indicating whether the AP can simultaneously support severalterminals when a terminal selects a to-be-accessed AP from a pluralityof candidate APs, and a terminal supporting the method.

The present invention also provides a method capable of collectivelyconsidering an amount of available radio resources when a terminalselects an AP, and a terminal supporting the method.

According to an aspect of the present invention, a method for providinginformation of access point (AP) selection in a wireless local areanetwork (WLAN) system, the method includes: receiving from candidate APsthe AP selection information comprising channel correlation informationfor other stations associated with the candidate APs; and selecting oneAP from candidate APs according to the channel correlation informationcorresponding to the candidate APs.

According to another aspect of the present invention, a method forproviding information of access point (AP) selection, the methodincludes: receiving from candidate APs the AP selection informationindicating radio resource utilization of other stations associated withthe candidate APs; selecting one AP from the candidate APs according tothe AP selection information.

According to embodiments of the present invention, a terminal can selectan access point (AP) capable of providing a better quality of service. Arelationship of other terminals currently accessed to the AP can betaken into account in addition to AP selection informationconventionally considered. In particular, a channel correlation can beconsidered to minimize factors of interference generated by otherterminals. Further, a gain of multi-user multiple input multiple output(MIMO) can be increased by associating simultaneously supportableterminals with the same AP.

In addition, an AP is selected by collectively considering not onlyinformation on channel utilization but also information suitable for amultiple-antenna environment, so that a suitable AP can be selected bycalculating an amount of actually available radio resources among allradio resources.

In addition, AP selection information is provided by directly usingconventional passive scanning or active scanning, thereby reducing anoverhead caused by generation of additional signaling.

In addition, a problem of omitting legacy stations in a determination orconsideration operation in a process of AP selection can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of a wireless local areanetwork (WLAN) system according to an embodiment of the presentinvention.

FIG. 2 shows a WLAN system in an overlapping basic services set (OBSS)environment capable of providing access point (AP) selection informationaccording to an embodiment of the present invention.

FIG. 3 is a flowchart showing a method for providing AP selectioninformation according to an embodiment of the present invention.

FIG. 4 is a flowchart showing a method for providing AP selectioninformation according to another embodiment of the present invention.

FIG. 5 is a flowchart showing a method for providing AP selectioninformation according to another embodiment of the present invention.

FIG. 6 is a flowchart showing a method for providing AP selectioninformation according to another embodiment of the present invention.

FIG. 7 shows a method for providing AP selection information accordingto another embodiment of the present invention.

FIG. 8 shows a frame format of AP selection information transmittedaccording to an embodiment of the present invention.

FIG. 9 shows a WLAN system capable of performing a method for providingAP selection information according to another embodiment of the presentinvention.

FIG. 10 and FIG. 11 show transmission opportunity (TXOP) of stations towhich the method for providing AP selection information according to theembodiment of FIG. 8 is applicable.

FIG. 12 is a flowchart showing a method for providing AP selectioninformation according to the embodiment of FIG. 9 to FIG. 11.

FIG. 13 shows an example of a frame format of AP selection informationprovided to a station in the method for providing the AP selectioninformation according to the embodiment described with reference to FIG.9 to FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.Although a wireless local area network (WLAN) system will be describedas an example in the embodiments described below among wirelesscommunication systems, this is for exemplary purposes only. Therefore,the following embodiments of the present invention can equally apply toother wireless communication systems in addition to the WLAN systemunless it is not allowed by nature. Terms or words used in the followingembodiments are unique to the WLAN system, but may be modified to othersuitable terms or words customarily used in the WLAN system.

FIG. 1 is a schematic view showing an example of a WLAN system accordingto an embodiment of the present invention.

Referring to FIG. 1, the WLAN system includes one or more basis servicesets (BSSs). The BSS is a set of stations (STAs) which are successfullysynchronized to communicate with one another, and is not a conceptindicating a specific region. The BSS can be classified into aninfrastructure BSS and an independent BSS (IBSS).

Infrastructure BSSs (i.e., BSS 1 and BSS 2) include one or more STAs,access points (APs) which are STAs providing a distribution service, anda distribution system (DS) associating a plurality of APs. On the otherhand, the IBSS does not include APs, and thus all STAs are mobile STAs.In addition, the IBSS constitutes a self-contained network since accessto the DS is not allowed. The embodiment of the present invention is notdirectly related to the IBSS.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer (PHY) interfaceconforming to the institute of electrical and electronics engineers(IEEE) 802.11 standard, and includes both an AP and a non-AP STA in abroad sense. The non-AP STA is an STA which is not an AP. For example,terminals operated by users are non-AP STAs (i.e., STA 1, STA 3, STA 4,STA 5. A non-AP STA may be simply referred to as an STA.

STA for wireless communication includes a processor and a transceiver,and also includes a user interface, a display element, etc. Theprocessor is a functional unit devised to generate a frame to betransmitted through a wireless network or to process a frame receivedthrough the wireless network, and performs various functions to controlSTAs. The transceiver is functionally associated with the processor andis a functional unit devised to transmit and receive a frame for theSTAs through the wireless network.

[The non-AP STA may also be referred to as a wireless transmit/receiveunit (WTRU), a user equipment (UE), a mobile station (MS), a mobileterminal, a mobile subscriber unit, etc.

The AP is a functional entity for providing connection to the DS througha wireless medium for an STA associated with the AP. Althoughcommunication between non-AP STAs in an infrastructure BSS including theAP is performed via the AP in principle, the non-AP STAs can performdirect communication when a direct link is set up. In addition to theterminology of an access point, the AP may also be referred to as acentralized controller, a base station (BS), a node-B, a basetransceiver system (BTS), a site controller, etc.

As described above, the STA can select the AP by performing a scanningprocess. There are two types of scanning. One is passive scanning, andthe other is active scanning. Information required by the STA for APselection can be transmitted by performing the passive scanning or theactive scanning. Hereinafter, the conventional passive scanning andactive scanning will be described in brief.

A first scheme is the passive scanning using a beacon frame transmittedfrom an AP. According to the passive scanning, a terminal which intendsto become a member of a specific BSS receives a beacon frameperiodically transmitted by the AP and thus can obtain a list ofcandidate APs and information on each candidate AP. In this case, thecandidate APs transmit a beacon frame including a service set ID (SSID)identical to an ID of a to-be-accessed BSS.

A second scheme is the active scanning. According to the activescanning, a terminal which intends to become a member of a specific BSSfirst transmits a probe request frame. The probe request frame includesa to-be-accessed SSID. Upon receiving the probe request frame, each APtransmits a probe response frame including a variety of information tothe terminal if its SSID coincides with the SSID included in thereceived probe request frame. Therefore, the terminal can obtain thelist of candidate APs by using the received probe response frame.

Additional information obtained to allow the STA to be able to selectthe AP according to the embodiment of the present invention may beincluded in any one field or an information element (IE) included in abeacon frame, a probe request frame, and/or a probe response frame. Theadditional information obtained for AP selection can be expressedwithout restriction.

FIG. 2 shows a WLAN system in an overlapping basic services set (OBSS)environment capable of providing AP selection information according toan embodiment of the present invention. In the OBSS environment, aplurality of BSSs are located in a specific region, and an overlappingregion exists between the BSSs.

The WLAN system of FIG. 2 includes a BSS 1 and a BSS 2, and has anoverlapping portion between them. An AP corresponding to the BSS 1 is anAP 1. An AP corresponding to the BSS 2 is an AP 2. The AP 1 and the AP 2are candidate APs. An STA 1, an STA 2, an STA 3, and an STA 4 exist inan overlapping region between the BSS 1 and the BSS 2. The STA 1 and theSTA 2 have a high channel correlation with each other. The STA 3 and theSTA 4 have a high channel correlation with each other.

The channel correlation between STAs may imply a degree of influenceexisting between channels corresponding to the respective STAs. Forexample, when a degree of influence is high since interference occurs ina channel of the STA 2 due to communication with the STA 1, the STA 1and the STA 2 have a high channel correlation with each other.

A multiple-antenna communication system includes link reliability orsystem capacity by using a transmit/receive diversity scheme, a spatialmultiplexing scheme, a beamforming scheme, etc., according to acharacteristic of a service and a channel environment. Such amultiple-antenna communication system is sensitive to a spatial channelenvironment. In case of using the diversity scheme, the lower thechannel correlation between links of each STA, the higher theperformance. In an actual channel environment, however, variouscorrelations exist between antennas, which may be significantlydifferent from an ideal case. And hereinafter, in the embodiment ofpresent invention, multiple-antenna communication system may mean acommunication system in which multiple spatial streams can betransmitted. And the ‘antenna’ in the present invention may be a virtualantenna. However, the scope of the present invention is not limited bythe meaning of the antenna, whether the antenna is a virtual antenna ora physical antenna.

In terms of distance, the STA 1 and the STA 2 are located close to theAP 1, and the STA 3 and the STA 4 are located close to the AP 2. In thiscase, however, the AP 1 cannot simultaneously provide services to theSTA 1 and the STA 2, and the AP 2 cannot simultaneously provide servicesto the STA 3 and the STA 4. Therefore, when providing services, the STA1 and the STA 3 are preferably associated with the AP 1, and the STA 2and the STA 4 are preferably associated with the AP 2.

That is, information to be basically considered in AP selection of STAsmay be information on a distance between the AP and the STA, a BSS load,channel quality, etc. Information on the BSS load includes informationon channel utilization. Information on a channel correlation betweenSTAs associated with the AP will be additionally considered in theembodiment of the present invention.

As such, in an AP selection mechanism, only STAs which cansimultaneously receive services from one AP are associated with the sameAP by considering a channel correlation with other STAs, and thus a gainof multi-user multiple input multiple output (MIMO) can be increased.

FIG. 3 is a flowchart showing a method for providing AP selectioninformation according to an embodiment of the present invention. Theembodiment of FIG. 3 can be achieved by active scanning.

An STA receives AP selection information from an AP (step 310). Herein,a candidate AP is an AP which receives the AP selection informationuntil the STA selects one AP. The AP selection information includeschannel correlation information of other STAs associated with the AP.The channel correlation information may include at least one of areference value for determining whether a channel correlation of otherSTAs is low or high, the number of STAs having a high correlationaccording to a determination result, a channel utilization of STAsdetermined to have a high channel correlation, etc.

Upon receiving the AP selection information from the AP, the STAdetermines whether to select the AP by using channel correlationinformation of the STAs associated with the AP (step 320). By using APselection information corresponding to several APs, the STA can possiblyselect an AP not associated with an STA having a high channelcorrelation, and then can access to the AP.

FIG. 4 is a flowchart showing a method for providing AP selectioninformation according to another embodiment of the present invention. Inan example described below, the AP selection information obtained by anSTA from an AP is channel correlation information of STAs.

First, the STA transmits a channel correlation information request tothe AP (step 410). The channel correlation information request may betransmitted to the STA by being included in a probe request. Further,the STA may announce the fact that the STA supports spatial divisionmultiple access (SDMA) by using a probe request frame. Herein, acandidate AP is an AP which receives the AP selection informationrequest from the STA or which transmits the AP selection information tothe STA until the STA selects one AP. The same is also applied in thefollowing description.

The probe request frame received by the AP may include a training field.Upon receiving the probe request frame including the training field, theAP performs channel estimation on a corresponding channel. The AP maygenerate information on a channel correlation according to a resultobtained by performing channel estimation.

In response to the channel correlation information request of the STA,the AP transmits a channel correlation information response to the STA(step 420). The channel correlation information response can also betransmitted to the STA by being included in the probe response. The APtransmits channel correlation information of other STAs associated withthe AP to the STA by using the channel correlation in-formationresponse.

That is, AP selection information depending on request of AP selectioninformation such as channel correlation information can be obtained bytransmitting a probe request and a probe response in an active scanprocess.

A terminal selects a to-be-accessed AP by using the received channelcorrelation information (step 430).

FIG. 5 is a flowchart showing a method for providing AP selectioninformation according to another embodiment of the present invention.The method for performing AP selection information according to theembodiment of the FIG. 5 can also be achieved by active scanning.

An STA transmits a probe request to an AP (step 510). The AP transmits aprobe response in response to the probe request, wherein the proberesponse transmitted by the AP includes a training request (step 520).

The STA transmits a sounding physical layer convergence procedure (PLCP)protocol data unit (PPDU) to the AP according to the training requestincluded in the probe response (step 530). The sounding PPDU is a PPDUtransmitted so that a transmitting side allows a receiving side to beable to estimate a channel between the transmitting side and thereceiving side. When a sounding parameter value of a transmission vectoror a reception vector is ‘sounding’, a PPDU used in that case isreferred to as a sounding PPDU.

After transmission of the probe request and the probe response, thesounding PPDU may be transmitted by using a training request included inthe probe response, or may be transmitted in a state where the proberequest is included in the sounding PPDU. A channel correlationinformation request is included in the sounding PPDU in any cases in theembodiment of the present invention.

Herein, when the STA transmits the sounding PPDU to the AP, the STAtransmits the sounding PPDU by including the channel correlationinformation request into the sounding PPDU. Then, while receiving thesounding PPDU, the AP transmits AP selection information including thechannel correlation information to the STA in response to the channelcorrelation information request of the STA (step 540). In anotherembodiment of the present invention, the channel correlation informationrequest and the sounding PPDU are separately transmitted. The channelcorrelation information request frame is first transmitted, followed bythe sounding PPDU.

FIG. 6 is a flowchart showing a method for providing AP selectioninformation according to another embodiment of the present invention. Anull data packet (NDP) is used in the method for providing AP selectioninformation according to the embodiment of FIG. 6. An AP is any one ofcandidate APs. An STA transmits a probe request frame to the AP (step610). The probe request frame may be broadcast. Upon receiving the proberequest from the STA, the AP responds to the STA by transmitting a proberesponse frame (step 620).

Upon receiving the probe response frame from the AP, the STA transmits achannel correlation information request to obtain channel correlationinformation from the AP (step 630). The channel correlation informationrequest may have a format of a channel correlation request managementaction frame.

The channel correlation request management action frame corresponds to apublic action frame. The public action frame is an action managementframe that can be transmitted and received before the STA and the AP areassociated with each other. Therefore, even in a state where the STA isnot associated with the AP, the STA can transmit and receive an actionmanagement frame (i.e., a request management action frame and a responsemanagement action frame to be described below) to and from the AP.

When the STA requests the AP to transmit the channel correlationinformation, a null data packet (NDP) announcement field may be set inan MAC header of the channel correlation management action frame. Thatis, an NDP announcement can be transmitted together with the channelcorrelation information request. For example, by setting the NDPannouncement field in a control field (e.g., a high throughput (HT)control field or a very high throughput (VHT) control field) of the MACheader, the STA announces to the AP that an NDP will be transmitted tothe AP subsequent to transmission of the channel correlation informationrequest.

The NDP is a PPDU in which only a PHY header is transmitted and in whicha physical layer service data unit (PSDU) is null.

Null data is one of MAC data types of IEEE 802.11. The null data hasneither a MAC header nor a data unit, and thus the null data has neithera sources address nor a destination address. Therefore, the null datahas to be continuously transmitted immediately subsequent totransmission of a non-NDP PPDU.

Herein, the non-NDP PPDU implies a general PPDU rather than an NDP. Ofcourse, a message for an AP selection information request or a channelcorrelation request management action frame, and a channel correlationresponse management action frame also correspond to the non-NDP PPDU.The NDP has to be transmitted immediately subsequent to transmission ofthe non-NDP PPDU, and in this case, an NDP announcement has to beincluded in the non-NDP PPDU transmitted prior to the transmission ofthe NDP. That is, the STA has to announce to the AP the fact that theNDP will be transmitted by using the NDP announcement immediately priorto the transmission of the NDP.

For example, according to 802.11m, the NDP announcement may be performedby setting a corresponding bit of the HT control field to 1. A sourceaddress and a destination address of the frame including the NDPannouncement are a source address and a destination address of the NDP.

A length field of a PLCP header is set to 0 in the NDP. It can be seenthat the NDP also corresponds to one sounding PPDU, and is used forchannel estimation. After NDP announcement, the STA transmits the NDP tothe AP (step 640).

Upon receiving the NDP, the AP estimates a channel between the STA andthe AP (step 650). The AP calculates a channel correlation for the STAaccording to a result of channel estimation, and generates channelcorrelation information to be provided to the STA.

Then, in response to the channel correlation request management actionframe received from the STA in step 630, the AP transmits the previouslygenerated channel correlation information to the STA (step 660). Thechannel correlation information can be transmitted in a form of achannel correlation response management action frame. The channelcorrelation response management action frame also corresponds to one ofpublic action frames, and channel correlation information is included inthe channel correlation response management action frame. Of course, thechannel correlation in-formation may be one example of AP selectioninformation provided by the AP to the STA or information included in theAP selection information.

FIG. 7 shows a method for providing AP selection information accordingto another embodiment of the present invention. The method for providingAP selection in-formation according to the embodiment of FIG. 7 can alsobe achieved by active scanning.

First, an STA transmits a probe request to an AP, wherein the proberequest includes a training request (step 710). In response to the proberequest of the STA, the AP transmits a probe response to the STA (step720). While receiving the probe response, the STA performs channelestimation on a downlink channel (step 730).

That is, a channel estimation request may be included in the proberesponse transmitted by the AT to the STA. The channel estimationrequest may correspond to a ‘training field’ transmitted from the STA tothe AP by being included in the probe request described above withreference to FIG. 4. Accordingly, the STA performs channel estimation.Unlike in the aforementioned embodiment, channel estimation is performedon the downlink channel directly by the STA not the AP.

Then, the STA transmits channel estimation information or channel stateinformation generated according to a result of the channel estimation tothe AP (step 740). Upon receiving the channel estimation informationfrom the STA, the AP calculates a channel correlation by using thereceived channel estimation information (step 750). Therefore, the APgenerates channel correlation information of the STA according to theresult of channel estimation. Then, the AP transmits the channelcorrelation information to the STA (step 760). The STA can select the APaccording to the channel correlation information (step 770).

FIG. 8 shows a frame format of AP selection information transmittedaccording to an embodiment of the present invention. The AP selectioninformation of FIG. 8 may be used as an example of channel correlationinformation of STAs as described above. A frame format of the channelcorrelation information may be referred to as a channel correlationinformation (CCI) frame format.

According to the CCI frame format shown in FIG. 8, the AP selectioninformation includes an Element ID field 810, a Length field 820, aChannel Correlation Threshold field 830, a Number of Correlated STAsfield 840, a Number of Uncorrelated STAs field 850, a ChannelUtilization of Correlated STA field 860, and a Channel Utilization ofUncorrelated STA field 870.

The Element ID field 810 may indicate that a frame is used fortransmission of channel correlation information. The Length field 820may indicate length information of the frame.

The Channel Correlation Threshold field 830 includes a channelcorrelation threshold which is a reference value for determining whethera channel correlation of a certain STA is high or low. The ChannelCorrelation Threshold field 830 may be included when a channelcorrelation of the STA is recognized by using a threshold. According toanother embodiment, an STA having a high correlation and an STA having alow correlation can be identified without having to use the threshold.

Hereinafter, for convenience of explanation, an STA having a highchannel correlation with respect to an STA intending to select the APmay be referred to as a correlated STA, and an STA having a low channelcorrelation may be referred to as an uncorrelated STA. Of course, incomparison with a channel correlation threshold of the ChannelCorrelation Threshold field 830, an STA having a higher channelcorrelation may be referred to as a correlated STA, and an STA having alower channel correlation may be referred to as an uncorrelated STA.

The Number of Correlated STAs field 840 indicates the number of STAswhich are determined to have a high channel correlation by a specificcriterion among STAs associated with the AP. Therefore, a quality ofservice may decrease when a large number of correlated STAs areassociated with the AP, wherein the number of correlated STAs isindicated in the Number of Correlated STAs field 840. On the contrary,the Number of Uncorrelated STAs field 850 indicates the number of STAsclassified into an STA having a low channel correlation by a specificcriterion among the STAs associated with the AP.

The Channel Utilization of Correlated STA field 860 indicates channelutilization of correlated STAs associated with the AP, and indicates howmany portions of all channels or all radio resources are occupied orused by the correlated STAs.

On the other hand, the Channel Utilization of Uncorrelated STA field 870indicates channel utilization of uncorrelated STAs associated with theAP, and indicates how many portions of all channels or all radioresources are occupied or used by the correlated STAs.

By including the Channel Utilization of Correlated STA field 860 and theChannel Utilization of Uncorrelated STA field 870, the STA can selectthe AP by considering not only channel quality, a BSS load, and channelcorrelations of other STAs but also channel utilization depending on thechannel correlation.

In addition thereto, information on MAC addresses of correlated STAs tobe associated with the AP may be included in a CCI frame. An MAC Addressof Correlated STA field is not shown in FIG. 8.

FIG. 9 shows a WLAN system capable of performing a method for providingAP selection information according to another embodiment of the presentinvention. An AP selection process performed by an STA 5 will bedescribed in brief with reference to FIG. 9.

The WLAN system of FIG. 9 shows an OBSS environment where an overlappingregion exists between a BSS 1 and a BSS 2. In the WLAN system of FIG. 9,there is an overlapping portion between the BSS 1 and the BSS 2. An AP 1is an AP corresponding the BSS 1. An AP 2 is an AP corresponding to theBSS 2. The AP 1 and the AP 2 are candidate APs.

STAs 1 to 5 are located in the overlapping portion between the BSS 1 andthe BSS 2. The STA 1, the STA 2, and the STA 3 are associated with theAP 1, and the STA 4 is associated with the AP 2. The STA 5 determineswhich AP will be selected between the AP 1 and the AP 2.

According to another embodiment of the present invention, the STA 5selects an AP by calculating an amount of radio resources that can bepractically used by the STA to determine in which case a larger amountof radio resources are used between a case where the STA 1, the STA 2,and the STA 3 use a radio resource of the AP 1 and a case where the STA4 uses a radio resource of the AP 2.

When the STA 5 determines which AP will be selected from the AP 1 andthe AP 2, the STA 5 receives AP selection information from the AP 1 andthe AP 2. Each of AP selection information may be transmitted by beingincluded in a beacon frame or a probe response frame which is broadcastor unicast by the AP 1 and the AP 2. That is, the STA 5 can obtain theAP selection information without having to request the AP 1 and the AP 2to transmit the AP selection information. Passive scanning is used in anAP selection method according to embodiments described with reference toFIG. 9 and subsequent Figures.

FIG. 10 and FIG. 11 show transmission opportunity (TXOP) of STAs towhich the method for providing AP selection information according to theembodiment of FIG. 8 is applicable. In TXOP shown in FIG. 10, each STAis related to an AP 1 within one unit time. In TXOP shown in FIG. 11,each STA is related to an AP 2 within one unit time.

In a case where the AP 1 supports a total of 3 spatial streams and theAP 1 is associated with three STA of an STA 1, an STA 2, and an STA 3,the AP 1 transmits one spatial stream to one STA. All of the threespatial streams are respectively allocated to the STA 1, the STA 2, andthe STA 3. Or the AP 1 supports a total of 3 multiple antennas and theAP 1 is associated with an STA 1, an STA 2, and an STA 3, all of thethree antennas are respectively allocated to the STA 1, the STA 2, andthe STA 3. A duration of the TXOP of the STA 1, the STA 2, and the STA 3occupies about 50% of any unit time. Therefore, for convenience, anamount of radio resources that can be used when an STA 4 is associatedwith the AP 1 among a total amount of radio resources that can besupported by the AP 1 can be indicated by 0.5. A channel utilization is0.5. A portion indicated by a slashed line corresponds to a total amount1010 of radio resources that can be used when an STA 5 selects the AP 1.

In a relation with the AP 2, a duration of TXOP of the STA 4 correspondsto a length of about 75% in any unit time shown in FIG. 11. A channelutilization is 0.75. Therefore, the TXOP of the STA 4 is longer than theTXOP of the STA 1, the STA 2, and the STA 3. That is, the STA 5 selectsthe AP 1 when only the channel utilization is simply compared, whereas aresult of AP selection may differ considering a total amount ofavailable radio resources when accessing to the AP 2.

In a case where the AP 2 supports a total of 3 spatial streams and onlythe STA 4 is associated with the AP 2, AP transmits only one spatialstream to the STA 4 among all available spatial stream of the AP 2 andthe remaining two spatial streams stay in an idle state. In other words,only one antenna may be allocated to the STA 4 among all antennas of theAP and the remaining two antennas stay in a state of an idle antenna. Aportion indicated by a slashed line corresponds to a total amount 1110of radio resources that can be used when the STA 5 selects the AP 2. Itcan be seen that the total amount 1110 of radio resources that can beused when the STA 5 selects the AP 2 is greater than the total amount1010 of radio resources that can be used when the STA 5 selects the AP1.

That is, although TXOP of STAs associated with the AP 1 is shorter thanTXOP of an STA associated with the AP 2, it can be seen that the STA 5preferably selects the AP 2 rather than the AP 1 by consideringbandwidth utilization and/or antenna utilization of the STAs.

FIG. 12 is a flowchart showing a method for providing AP selectioninformation according to the embodiment of FIG. 9 to FIG. 11.

An STA receives AP selection information from neighboring candidate APs.Herein, an STA for selecting an AP is the STA 5 of FIG. 9 and FIG. 11,and the candidate APs are the AP 1 and the AP 2. Therefore, the STA 5receives the AP selection information from the AP 1 and the AP 2 (step1210 and step 1220). The AP selection information may be transmitted bybeing included in a beacon frame or a probe response frame.

The AP 1 and the AP 2 broadcast or unicast a beacon frame or a proberesponse frame. The STA 5 can obtain AP selection information on the AP1 and the AP 2 by receiving the beacon frame or the probe response framefrom the AP 1 and the AP 2.

The AP select information received by the STA 5 from the AP 1 and the AP2 in the embodiment of FIG. 12 is different to some extent from channelcorrelation in-formation described in the previous embodiment. The APselection information includes bandwidth utilization information orantenna utilization information of STAs associated with one AP. Thebandwidth utilization information is information indicating a length ofa channel bandwidth currently used among a full channel bandwidth.

That is, the AP 1 transmits to the STA 5 the antenna utilizationinformation of STAs (i.e., STA 1, STA 2, and STA 3) associated with theAP 1, and the AP 2 transmits to the STA 5 the antenna utilizationinformation of an STA (i.e., STA 4) associated with the AP 2. Of course,the AP selection information transmitted by the AP 1 and the AP 2 to theSTA 5 may include channel utilization of STAs associated with each AP.As described above, the AP selection information according to theembodiment of the present invention may further include information on achannel bandwidth used by STAs associated with a corresponding AP.

Upon receiving the AP selection information from the AP 1 and the AP 2,the STA 5 calculates an amount of radio resources that can be used whenassociated with the AP 1 and the AP 2 by using the antenna utilizationinformation or the bandwidth utilization information of the AP 1 and theAP 2 (step 1230). The calculation process has been described withreference to FIG. 10 and FIG. 11. In the present embodiment, the STA 5selects the AP 2 by determining that an amount of radio resources whichcan be used when associated with the AP 2 is greater as a result of thecalculation process (step 1240).

FIG. 13 shows an example of a frame format of AP selection informationprovided to an STA in the method for providing the AP selectioninformation according to the embodiment described with reference to FIG.9 to FIG. 11.

According to the frame format of the AP selection information shown inFIG. 13, the AP selection information includes an Element ID field 1310,a Length field 1320, a Number of STAs field 1330, a Number of LegacySTAs field 1340, a Channel Utilization of STAs field 1350, a ChannelUtilization of Legacy STAs field 1360, an Antenna Utilization of STAsfield 1370, a Bandwidth Utilization of STAs field 1380, etc.

Herein, in case of using a legacy STA, multiple antennas are notsupported, and a supported channel bandwidth may be narrower than thoseof general STAs not the legacy STA. Therefore, STAs are differentiatedfrom the legacy STA in the present embodiment. For example, it can beunderstood that the STA denotes a VHT STA of a VHT WLAN system, and thelegacy STA is a non-VHT STA of the VHT WLAN system.

The VHT WLAN system has been proposed to meet a demand on a new WLANsystem supporting a higher throughput than a data processing speedsupported by IEEE 802.11n. The VHT WLAN system is one of IEEE 802.11WLAN systems which have recently been proposed to support a dataprocessing speed of 1 Gbps or more. The VHT WLAN system is namedarbitrarily. To provide a throughput of 1 Gbps or more, a feasibilitytest is currently being conducted for a system which uses 4?4 MIMO and achannel bandwidth of 80 MHz or more.

For example, in the VHT WLAN system, the VHT STAs support a channelbandwidth of 80 MHz and multiple spatial streams or multiple antennas.However, the non-VHT STA that is the legacy STA may support only asingle spatial stream or single antenna with a channel bandwidth of 20MHz. Therefore, for convenience of explanation, the legacy STA isdifferentiated when explaining the present embodiment in which bandwidthutilization and antenna utilization are used as AP selectioninformation.

The Element ID field 1310 and the Length field 1320 respectivelyindicate a purpose or role of a frame and a length of the frame.

The Number of STAs field 1330 indicates the number of STAs associatedwith a corresponding AP. In the VHT WLAN system for example, the Numberof STAs field 1330 indicates the number of VHT STAs associated with acorresponding VHT AP. Further, the Number of Legacy STAs field 1340indicates the number of legacy STAs associated with a corresponding AP.

In the VHT WLAN system for example, a time in which multiple spatialstreams or multiple antennas can be used decreases when the number ofnon-VHT STAs that are legacy STAs increases, and a supported bandwidthdecreases during a time in which the AP communicates with the legacySTA. Therefore, the number of legacy STAs can also be informationrequired to calculate a total amount of radio resources that can be usedwhen a specific AP is selected by an STA intending to select the AP.

The Channel Utilization of STAs field 1350 indicates channel utilizationof STAs associated with the AP. The STA intending to select the AP canknow which channel is used by each STA associated with the AP and canknow a channel which is already occupied by other STAs by using theChannel Utilization of STAs field 1350.

The Channel Utilization of Legacy STAs field 1360 indicates channelutilization of legacy STAs associated with the AP. The STA intending toselect the AP can know a specific channel used by the legacy STAs whichare already associated with the AP and can know a specific channel andhow many portions of the channel are occupied and used.

The Antenna Utilization of STAs field 1370 may indicate which spatialstream is already allocated to the other STAs communicating with the AP.The Antenna Utilization of STAs field 1370 may also indicate a specificantenna through which other STAs already associated with a correspondingAP communicate with the AP among several antennas of the AP. Forexample, if the AP is a VHT AP, since the VHT AP supports amultiple-spatial stream system, a spatial stream in use may coexist witha spatial stream that can be used by an STA to be newly associated.Therefore, the STA recognizes antenna utilization of candidate antennasor available spatial stream, and thus can select the AP by consideringwhich spatial stream can be used in an idle mode and by consideringwhich spatial stream is already being used. Likewise, the STA can selectthe AP by considering which antenna or how many antennas can be used inan idle mode and by considering which antenna is already being used.

The Bandwidth Utilization of STAs field 1380 can also be explained bytaking a case where the AP is the VHT AP for example. When the APsupports a wide frequency band, several STAs can share a channelbandwidth by being simultaneously associated with the AP. Therefore,some portions of a full channel bandwidth are in use, and the remainingportions thereof are in the idle mode. Accordingly, the STA intending toselect the AP can obtain information indicating a length of a channelbandwidth not in use at the moment or information indicating to whichband the channel bandwidth belongs from the Bandwidth Utilization ofSTAs field 1380, and thus can apply the obtained information to the APselection process.

All functions described above may be performed by a processor such as amicroprocessor, a controller, a microcontroller, an application specificintegrated circuit (ASIC) or a processor of a terminal according tosoftware or program code for performing the functions. The program codemay be designed, developed, and implemented on the basis of thedescriptions of the present invention, and this is well known to thoseskilled in the art.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method of transmitting basic service set (BSS)load information in a wireless local area network, the methodcomprising: establishing, by an access point (AP), a BSS which is a setof member stations that have successfully synchronized with the AP andthat have a capability of receiving at least one spatial stream of aplurality of spatial streams transmitted via a multi user-multiple inputmultiple output (MU-MIMO) transmission; and transmitting, by the AP, abeacon frame to a station, the beacon frame including load informationincluding a station count field and an antenna utilization field,wherein the station count field indicates a number of the memberstations that have successfully synchronized with the AP and that havethe capability of receiving the at least one spatial stream of theplurality of spatial streams transmitted via the MU-MIMO transmission,and wherein the antenna utilization field indicates information aboutspatial streams that are under-utilized in the MU-MIMO transmission. 2.The method of claim 1, wherein the load information further includes abandwidth utilization field indicating information about bandwidthutilized when the AP performs MU-MIMO transmission.
 3. The method ofclaim 1, wherein the station count field has one octet in length and theantenna utilization field has one octet in length.
 4. A deviceconfigured to transmit basic service set (BSS) load information in awireless local area network, the device comprising: a communicationdevice; and a processor configured to: establish a BSS which is a set ofmember stations that have successfully synchronized with the device andthat have a capability of receiving at least one spatial stream of aplurality of spatial streams transmitted via a multi user-multiple inputmultiple output (MU-MIMO) transmission, and instruct the communicationdevice to transmit a beacon frame to a station, the beacon frameincluding load information including a station count field and anantenna utilization field, wherein the station count field indicates anumber of the member stations that have successfully synchronized withthe device and that have the capability of receiving at least onespatial stream of the plurality of spatial streams transmitted via theMU-MIMO transmission, and wherein the antenna utilization fieldindicates information about spatial streams that are under-utilized inthe MU-MIMO transmission.
 5. The device of claim 4, wherein the loadinformation of the BSS further includes the bandwidth utilization fieldindicating information about bandwidth utilized when the device performsMU-MIMO transmission.
 6. The device of claim 4, wherein the stationcount field has one octet in length and the antenna utilization fieldhas one octet in length.
 7. A method of selecting an access point (AP)in a wireless local area network system, the method comprising:receiving, by a scanning station from an access point (AP), a beaconframe including load information including a station count field and anantenna utilization field; and determining, by the a scanning stationbased on the load information, whether the station requests associationwith the AP or not, wherein the station count field indicates a numberof stations that have successfully synchronized with the AP and thathave a capability of receiving at least one spatial stream of aplurality of spatial streams transmitted via a multi user-multiple inputmultiple output (MU-MIMO) transmission, and wherein the antennautilization field indicates information about spatial streams that areunder-utilized in the MU-MIMO transmission.
 8. The method of claim 7,wherein the load information further includes the bandwidth utilizationfield indicating information about bandwidth utilized when the APperforms MU-MIMO transmission.
 9. The method of claim 7, wherein thestation count field has one octet in length and the antenna utilizationfield has one octet in length.
 10. A device configured to select anaccess point (AP) in a wireless local area network system, the devicecomprising: a communication device; and a processor configured to:instruct the communication device to receive, from an access point (AP),a beacon frame including load information including a station countfield and an antenna utilization field, and determine, based on the loadinformation, whether the station requests association with AP or not,wherein the station count field indicates a number of stations that havesuccessfully synchronized with the AP and that have a capability ofreceiving at least one spatial stream of a plurality of spatial streamstransmitted via a multi user-multiple input multiple output (MU-MIMO)transmission, and wherein the antenna utilization field indicatesinformation about spatial streams that are under-utilized in the MU-MIMOtransmission.
 11. The device of claim 10, wherein the load informationfurther includes the bandwidth utilization field indicating informationabout bandwidth utilized when the AP performs MU-MIMO transmission. 12.The device of claim 10, wherein the station count field has one octet inlength and the antenna utilization field has one octet in length.